We propose to clarify the taxonomic relationships of several periodontal bacteria by comparng the base sequences of their deoxyribonucleic acids (DNA). DNA will be extracted from cells lysed by combinations of penicillin, lysozyme, and sodium lauryl sulfate. The molar ratio of guanine and cytosine in the total DNA bases will be determined by thermal denaturation. DNA base sequence similarities will be assessed by hybrid reassociation on membrane filters, and in solution through the use of single-strand specific nuclease(s). Organisms sharing 80% or more common sequences will be considered to be the same species. Below 50%, organisms will not be considered as members of the same species. Between 50 and 80%, other taxonomic criteria, such as phenotypic similarity and pathogenic similarity, will be considered in determining taxonomic position. We propose to study specific strains resembling Eikenella, Bacteroides, Capnocytophaga, and Fusobacterium.